Regular Expression to reformat a US phone number in Javascript

Question

Assuming you want the format "(123) 456-7890":

function formatPhoneNumber(phoneNumberString) {
  var cleaned = ('' + phoneNumberString).replace(/\D/g, '');
  var match = cleaned.match(/^(\d{3})(\d{3})(\d{4})$/);
  if (match) {
    return '(' + match[1] + ') ' + match[2] + '-' + match[3];
  }
  return null;
}

Here's a version that allows the optional +1 international code:

function formatPhoneNumber(phoneNumberString) {
  var cleaned = ('' + phoneNumberString).replace(/\D/g, '');
  var match = cleaned.match(/^(1|)?(\d{3})(\d{3})(\d{4})$/);
  if (match) {
    var intlCode = (match[1] ? '+1 ' : '');
    return [intlCode, '(', match[2], ') ', match[3], '-', match[4]].join('');
  }
  return null;
}
formatPhoneNumber('+12345678900') // => "+1 (234) 567-8900"
formatPhoneNumber('2345678900')   // => "(234) 567-8900"

Possible solution:

function normalize(phone) {
    //normalize string and remove all unnecessary characters
    phone = phone.replace(/[^\d]/g, "");

    //check if number length equals to 10
    if (phone.length == 10) {
        //reformat and return phone number
        return phone.replace(/(\d{3})(\d{3})(\d{4})/, "($1) $2-$3");
    }

    return null;
}

var phone = '(123)4567890';
phone = normalize(phone); //(123) 456-7890

var x = '301.474.4062';
    
x = x.replace(/\D+/g, '')
     .replace(/(\d{3})(\d{3})(\d{4})/, '($1) $2-$3');

alert(x);

This answer borrows from maerics' answer. It differs primarily in that it accepts partially entered phone numbers and formats the parts that have been entered.

phone = value.replace(/\D/g, '');
const match = phone.match(/^(\d{1,3})(\d{0,3})(\d{0,4})$/);
if (match) {
  phone = `${match[1]}${match[2] ? ' ' : ''}${match[2]}${match[3] ? '-' : ''}${match[3]}`;
}
return phone

I'm using this function to format US numbers.

function formatUsPhone(phone) {

    var phoneTest = new RegExp(/^((\+1)|1)? ?\(?(\d{3})\)?[ .-]?(\d{3})[ .-]?(\d{4})( ?(ext\.? ?|x)(\d*))?$/);

    phone = phone.trim();
    var results = phoneTest.exec(phone);
    if (results !== null && results.length > 8) {

        return "(" + results[3] + ") " + results[4] + "-" + results[5] + (typeof results[8] !== "undefined" ? " x" + results[8] : "");

    }
    else {
         return phone;
    }
}

It accepts almost all imaginable ways of writing a US phone number. The result is formatted to a standard form of (987) 654-3210 x123

thinking backwards

Take the last digits only (up to 10) ignoring first "1".

function formatUSNumber(entry = '') {
  const match = entry
    .replace(/\D+/g, '').replace(/^1/, '')
    .match(/([^\d]*\d[^\d]*){1,10}$/)[0]
  const part1 = match.length > 2 ? `(${match.substring(0,3)})` : match
  const part2 = match.length > 3 ? ` ${match.substring(3, 6)}` : ''
  const part3 = match.length > 6 ? `-${match.substring(6, 10)}` : ''    
  return `${part1}${part2}${part3}`
}

example input / output as you type

formatUSNumber('+1333')
// (333)

formatUSNumber('333')
// (333)

formatUSNumber('333444')
// (333) 444

formatUSNumber('3334445555')
// (333) 444-5555

2021

libphonenumber-js

Example

import parsePhoneNumber from 'libphonenumber-js'

const phoneNumber = parsePhoneNumber('+12133734253')

phoneNumber.formatInternational() === '+1 213 373 4253'
phoneNumber.formatNational() === '(213) 373-4253'
phoneNumber.getURI() === 'tel:+12133734253'